As an optical fiber in which Rayleigh scattering is small and transmission loss is small, an optical fiber of silica-based glass in which an alkali metal element is doped to its core is known. If a core portion (core preform) of an optical fiber preform includes the alkali metal element, viscosity of the core portion can be decreased when the optical fiber preform is drawn. In this case, since a network structure of the silica glass is relaxed, a fictive temperature in the optical fiber decreases. As a result, the alkali metal element is doped to the core, so that reduction of the transmission loss in the optical fiber is theoretically enabled.
As a method of doping the alkali metal element to the silica glass, a diffusion method is known (for example, refer to PTLs 1 and 2). In the diffusion method, for diffusing and doping the alkali metal element diffuses into an inner surface of a glass pipe, raw material vapor of the alkali metal element or alkali metal salt to be a raw material is introduced into the glass pipe while the glass pipe is heated by an external heat source or plasma is generated in the glass pipe.
After the alkali metal element is doped to the vicinity of the inner surface of the glass pipe as described above, a diameter of the glass pipe is reduced by heating. After the diameter is reduced, the inner surface of the glass pipe is etched by a certain thickness for the purpose of removing a transition metal element such as Ni and Fe doped simultaneously in the doping of the alkali metal element. Diffusion of the alkali metal element is faster than diffusion of the transition metal element. For this reason, even though the glass surface is etched by the certain thickness (the transition metal element is removed), the alkali metal element can be caused to remain in the glass pipe. After the etching, the glass pipe is solidified by the heating, so that an alkali metal element doped core rod is manufactured. A cladding portion having a refractive index lower than that of the core portion including the alkali metal element doped core rod is synthesized to the outside of the alkali metal element doped core rod, so that an optical fiber preform is manufactured. Furthermore, the optical fiber preform is drawn, so that an optical fiber is manufactured.